In mobile telecommunications networks, there is a requirement for User Equipment (UE), that is under radio coverage, always to be able to make emergency calls, even when the UE has no (U)SIM card or when registration of the UE to a network has failed. Provision must, therefore, be made within the mobile communications networks to allow UEs to make such emergency calls.
The 3GPP has specified that the E-UTRA network will be a packet switched (PS) network only, so no circuit switched (CS) services will be implemented. This means that voice traffic has to be performed using Voice over IP (VoIP). However, due to the costs involved in implementing VoIP, 3GPP has considered that E-UTRAN may not support VoIP in Release-8 and envisages making it optional in subsequent releases. Therefore, to provide voice call services to UEs camped on an LTE cell, 3GPP has defined a mechanism that redirects the voice call request to a cell that supports CS calls. This mechanism is called CS Fallback and is described in 3GPP standards document TS 23.272. In essence this mechanism allows the UE to request a change of cell to one that supports voice services—typically to a GSM cell or a UTRA cell which support the CS domain.
As an emergency call is a voice call, it is envisaged that this CS Fallback mechanism will be used to establish the emergency call. However, the inventor has realized that this will cause a number of problems. Firstly, the current CS Fallback procedure requires the UE to be registered in NORMAL SERVICE before it can be used. This is because it requires the UE to send a “Serving Request” to the E-UTRA network and when the UE is registered with the cell in LIMITED SERVICE, the network is configured to reject such requests. Therefore, with the current CS Fallback procedure, emergency calls cannot be made when the UE is in LIMITED SERVICE mode. Further, even if the CS Fallback procedure were to be amended to allow the cell to accept the request, other technical problems remain. In particular, CS Fallback requires the implementation of inter-RAT mobility procedures which require the exchange of additional messages between the UE and the network and, in some cases, some UE measurements to be made. These additional procedures require additional signaling overhead, can increase the likelihood of emergency call failure and introduce a significant delay for emergency call establishment.